Introduction to the command line

Decklin Foster
RailsBridge Boston
September 20, 2014

Good morning!

This weekend, we will be using the command line to run Ruby while we learn to program.

It's probably a little different from how you're used to using your computer, but:

Read Along

Open this web page:

Getting started

Let's all open up a terminal (command line window) to follow along, if you don't already have one open from installing stuff.


That's why the different apps (Mac/Linux vs. Windows) have different names. It's not an important distinction for us. We'll just refer to them together as the command line.

How will this work?

When we have a command to run, I'll switch over to another desktop and type it in along with you.

Let's try it now so we can make sure the colors and fonts are readable!

Do you see what I see?

The last line "printed" to the terminal should look something like:

[email protected]:~$

The thing that ends in $ is the shell's prompt. It means it's waiting for your input.

Everything to the left of the $ is just information. You can ignore it for now.

Run a command!

Type this and press Return or Enter:


Everyone will see a different response.

How did that work?

To explain this, let's try a second command.

Type this and press Return or Enter:


What did we just do?

The current directory is like where your Finder (Mac) or Explorer (Windows) window is, or what page you're on in a web browser.

You can navigate down (into a directory) or up (to the containing, or "parent" directory), or to a totally new directory.

Try it!

When you ran ls, you probably saw Desktop. That's a directory. (If you didn't, ask a TA!).

Type this and press Return or Enter:

cd Desktop

In the desktop folder

From now on, I'll just say "run" for new commands.


mkdir example_folder

You should see the example_folder both on your computer's desktop and in the output of ls.


This brings up an important principle of many commands:

I'll show you what happens if the directory already exists.

Going to our new directory


cd example_folder

Nothing there yet!

Creating a file

Let's create a file for later. Run:

touch testing_the_command_line.txt

touch creates empty files. ls will display the file you just created.

Going back up


cd ..

.. goes "up" one directory. Now we're back in Desktop.

Going back up again

Run the same thing again:

cd ..

Now we're in your home directory, where you started.

Repeating commands

We ran that command again by typing it in. A quicker way to do this is to press the up arrow.

Try pressing the up and down arrows. You can go through all the history of commands you have typed.

Run cd .. again this way. Run it a few more times.

Now we're outside of your home directory, so let's go back.


If you ever get lost, you can run:

cd ~

or just


To get back to your home directory. This works from anywhere. ~ is the tilde key. On US keyboards, it's on the top left, next the 1 key, and you have to press SHIFT along with it.

There and back again

Try it! Run:

cd Desktop/example_folder
cd ~

That's a forward slash. It separates directories.

Files and Paths

The command line relies heavily on locating files via their names. This is called a path.

We used a path from your home directory to example_folder: (this is not a command)


Files and Paths (continued)

We can also give this path to other commands. Run:

ls Desktop/example_folder

A path allows you to give the command a file or directory without cding to it first.

Commands vs. arguments

In this command:

ls Desktop/example_folder

The first word, ls is the command. The computer finds the ls program and runs it.

The first word of a command is always the program.

Words after that are passed to the program as arguments. For ls, the arguments are what files or directories to list.

Saving some typing

Another way to save typing is with Tab-completion. When you are typing in a path, press TAB and the shell will try to complete it for you.

Run the same command again, but this time, type ls wor and press TAB. Then type r and press TAB again.

In addition to reducing the amount you have to type, tab-completion lets you make sure a file or directory exists before you run a command on it.

Completing commands

You can also complete the names of commands. Type p and press TAB. You will see something like:

Display all 203 possibilities? (y or n)

Press 'y'.

How does it know when to complete a command and when to complete a path? The first word is always the command. Arguments are typically paths.

For paths, completion will also ask you when there are multiple files that start with what you've typed.

How to get help

You can read the "manual pages" for commands with the man command. Run:

man ls

Here ls is not a command, but the argument to the command man.

man will bring up a "pager" that takes up your whole terminal. Scroll with the arrow keys or spacebar, and press q to quit.

Quiz time!

If you're feeling confident, try answering this. The command rm (short for remove) deletes files. The arguments are paths to files to delete. How could we clean up by deleting the testing_the_command_line.txt file?


This will work:

rm Desktop/example_folder/testing_the_command_line.txt

So will this:

cd Desktop
cd example_folder
rm testing_the_command_line.txt

There are always lots of ways to do things.

Absolute paths

What if I want a command to work from anywhere? A path to a file can also be absolute. Run:

ls ~/Desktop/example_folder

What does ls actually see here? We can use echo, which is a command that just outputs ("echoes") its arguments:

echo ~/Desktop/example_folder

You will see the full path, starting with /home or /Users. A path starting with a / works from anywhere. The shell expanded it for you before running echo.


You can create files by taking a command's output and redirecting it to a file instead of printing it to the screen. Run:

ls -l > list.txt

To output a file, run:

cat list.txt

(You may notice that list.txt contains itself!)

The Zen of the Command line

Where does the name cat come from? This is a bit of obscure trivia, but instructive.

cat is short for concatenate. If you give it more than one file, it will print all of them, in order.

Because there's a command to add together any number of files, there doesn't need to be one for only a single file.

This is a good goal to keep in mind when programming: design programs that are general, so that specific solutions don't require more code.


Some arguments to commands are called options. They start with a dash: -.

Try this:

ls -l
ls -a
ls -l -a

l stands for long format, and a stands for all files and directories.

You don't need to memorize these; you can look them up with man. But it's helpful to be able to recognize them.

Commands as language

I like to call the command line a language-based interface.

With that in mind, here's an example of the power language gives you:

My favorite language interface

Tea. Earl Grey. Hot.


If we have some time, I can go into a history lesson.

If not, I'll try to take some questions!

Some history

Why is it called a "terminal"? Well, this is what they used to look like:

WACs at teletypes

These things, called "teletypes", would be hooked up to computers the size of a room.

More history

Eventually everyone ran out of paper these were replaced with electronic terminals:

the venerable VT220

Same idea. No "computer" there, just a screen and keyboard. Where your line to the big computer "terminates".

Why CLI?




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